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Selected Rheological Properties of Some Tomato Ketchups 7th TAE 2019 17 - 20 September 2019, Prague, Czech Republic SELECTED RHEOLOGICAL PROPERTIES OF SOME TOMATO KETCHUPS Peter HLAVÁČ1, Monika BOŽIKOVÁ1, Zuzana HLAVÁČOVÁ1 1Department of Physics, Faculty of Engineering, Slovak University of Agriculture in Nitra Abstract This article is focused on monitoring and evaluation of rheological properties of tomato ketchups. Influence of temperature in range (5 – 30) °C and short storage time was investigated. Dependencies of rheological properties on temperature and storage time were evaluated by the regression equations and the coefficients of determination. Ketchup is a non-Newtonian material, so apparent viscosity was measured using digital rotational viscometer Anton Paar DV-3P. Densities of samples were deter- mined according to definition. Apparent viscosity of samples decreased exponentially with increasing temperature, so the Arrhenius equation is valid. Ketchup’s apparent fluidity was increasing exponen- tially with the temperature. We also found out that apparent viscosity had decreased with storage time and on the other hand apparent fluidity was increasing with storage time, which can be caused by structural changes in samples during storing. Temperature dependencies of ketchup densities were sufficiently characterized by decreasing linear function in measured temperature range. Key words: ketchup; rheological parameters; density; temperature, dependency. INTRODUCTION Precise knowledge of physical quantities of materials is required at controlled processes in manufac- turing, handling and holding. For the quality evaluation of food materials it is important to know their physical properties particularly, mechanical (Kubík et al., 2017), rheological and thermophysical (Glicerina et al., 2013). Tomatoes are often consumed in fresh state, but several varieties are pro- cessed into various products such as tomato sauce, soup, paste, puree, juice, ketchup and salsa. Au- thors noted that several studies had investigated the benefits of tomatoes in reducing the risk of heart disease, improving bone health, and decreasing the risk of cancer (Tan & Kerr, 2015). The industrial processing of tomatoes leads to a great variety of output products. Some of the most relevant are the following: concentrated tomato products, either as puree or paste depending on the percentage of natu- ral soluble solids; pizza sauce, from peels and seeds; tomato powder, as dehydrated concentrated to- mato; peeled tomato, either whole or diced; ketchup, tomato sauce seasoned with vinegar, sugar, salt and some spices, etc. (Ruiz Celma et al., 2009). New design of experimental double piston filament stretching apparatus that can stretch fluids to very high extensional strain rates was presented by Mackley et al. (2017). Authors had used high speed photography for determination of filament defor- mation and breakup profiles of a strategically selected range of fluids including low and higher viscos- ity Newtonian liquids together with a viscoelastic polymer solution, biological and yield stress fluids at extensional strain rates in excess of 1000 s−1. Mackley et al. (2017) had also reported that numerical modelling can be used with the fluids correct rheological characterization to gain physical in-sight into how rheologically complex fluids deform and breakup at very high extensional deformation rates. Chandrapala et al. (2012) had stated that ultrasonic pulses in combination with rheological measure- ments could be used at the solids concentration determination in various highly concentrated and in- dustrial food suspensions (tomato, vegetable and pasta sauces, seafood chowder, strawberry yoghurt, cheese sauce with vegetables). Low intensity ultrasound-based techniques could be used during the detection and identification of foreign bodies in food products (e.g. tomato ketchup) (Chandrapala et al., 2012). The rheological behaviour of some food dispersions were investigated and modelled with the Herschel–Bulkley model by Herrmann et al. (2013). Several rheologic parameters were studied in dependency on composition of various ingredients (fats, carbohydrates, proteins, water). The effects of manothermosonication or thermal treatment on tomato pectic enzymes and tomato paste rheological properties were compared by Vercet et al. (2002). The quality of tomato concentrate was studied by Fadavi et al. (2018) using ohmic vacuum, ohmic, and conventional-vacuum heating methods. Authors had indicated that ohmic heating under vacuum condition had a good effect on quality during the con- centration of tomato juice and that vacuum condition decreased the time required for processing and 165 7th TAE 2019 17 - 20 September 2019, Prague, Czech Republic heating rate. Small amplitude oscillatory shear (SAOS) and large amplitude oscillatory shear (LAOS) behaviour of tomato paste was investigated by Duvarci et al. (2017). Authors had stated that the semi- empirical Bird-Carreau constitutive model can be used in small amplitude oscillatory shear behaviour of tomato paste. Authors also used Ewoldt-McKinley theory for determination of non-linear rheologic properties in large amplitude oscillatory shear behaviour of tomato paste. This method offered parame- ters which can be used for better understanding of structural changes which occur at different defor- mations or time scales (Duvarci et al., 2017). Rheological properties of tomato products are consid- ered as one of the most important quality attributes, since they influence product processing parame- ters, especially flow properties during transport, as well as consumers’ acceptability (Torbica et al., 2016). Investigation of rheological properties and microstructure of tomato puree using continuous high pressure homogenization was done by Tan & Kerr (2015). Influence of rheological and structural characterization of tomato paste on the quality of ketchup was investigated by Bayod et al. (2008). According to Sharoba et al. (2005) ketchup is time-independent, semi-solid non-Newtonian fluid hav- ing a definite yield stress. The effect of temperature on viscosity of fluids at a specified shear rate could be described by the Arrhenius equation, in which the apparent viscosity decreases as an expo- nential function with temperature (Sharoba et al., 2005). According to Bayod et al. (2008) viscosity of tomato ketchup is a major quality component for consumer acceptance. Several parameters affect the flow behaviour of tomato ketchup, including the quality of the raw material (e.g. tomato paste) and the processing conditions. To achieve a constant and desirable quality in the final product (i.e. ketchup), high quality paste and continuous control and adjustment of the variables for its processing are re- quired (Bayod et al., 2008). Tomato ketchup could be presented as concentrated dispersion of insolu- ble matter in aqueous media, and its complex structure causes that it exhibits non-Newtonian, shear- thinning and time-dependent rheological behaviour with yield stress (Torbica et al., 2016). Ketchup can be included into one of the most commonly consumed condiment which is made either from fresh tomatoes or from the concentrates such as tomato purees and tomato pastes (Mert, 2012). Effect of temperature and concentration on rheological properties of ketchup–processed cheese mixtures was analysed using steady and dynamic oscillatory shear by Yilmaz et al. (2011). Temperature dependency of the apparent viscosity at a specified shear rate (50 s-1) could be described by the Arrhenius model (Yilmaz et al., 2011). Torbica et al. (2016) have evaluated nutritional, rheological, and sensory proper- ties of tomato ketchup with increased content of natural fibres made from fresh tomato pomace and compared the results with five commercial products. Authors found out that the rheological properties of the ketchup with increased fibre content depend mostly on total solids and insoluble particles con- tent, but properties remained in the limits for standard tomato products. Apparent viscosity is defined as ratio of shear stress and corresponding shear rate and its physical unit is (Pa·s). Viscosity changes with temperature, for most of the liquids decreases with increasing tem- perature. According to Eyering theory molecules of liquids continuously move into the vacancies (Bird et al., 1960). The temperature effect on viscosity can be described by an Arrhenius type equation E A e RT 0 (1) where 0 is reference value of viscosity, EA is activation energy, R is gas constant and T is absolute temperature (Figura & Teixeira, 2007). Density of material ρ is defined as a ratio between mass of material m and its volume V m V (2) The definition is valid for solids, liquids, gases and disperses (Figura & Teixeira, 2007). The standard SI unit of density is (kg·m-3). Reciprocal value of apparent viscosity η is called apparent fluidity φ and unit of apparent fluidity is (Pa-1.s-1) (Figura & Teixeira, 2007). 1 (3) Almost all parameters are influenced by temperature, so mainly these effects were analysed as a main aim in this article. 166 7th TAE 2019 17 - 20 September 2019, Prague, Czech Republic MATERIALS AND METHODS Measurements were performed in laboratory settings on four samples of tomato ketchup, purchased in local market (Heinz tomato ketchup, soft ketchup Hellmann´s, sweet ketchup Hamé and soft ketchup Snico). As ketchup is not Newtonian material, apparent viscosity had to be measured. Density (Eq. 2) and apparent fluidity (Eq. 3) were also determined. The measurements of all samples were carried
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